Risk factors for respiratory distress syndrome (RDS) in the newborn have been evaluated using data from a large survey conducted between 1980 and 1989 in selected periods in eleven perinatal units placed in five Italian regions. A total of 1624 liveborn infants consecutively delivered at the collaborating centers, at delivery 26-37 weeks gestational age and without clinically evident congenital anomalies were included in the survey. All the newborns were followed up to the 28th day of life. A total of 131 newborns (7.8%) developed RDS. Overall 1st-7th and 1st-28th day of life infant mortality rates were 54.8 and 61.6/1,000 livebirths; the corresponding rates in babies who developed RDS were 419.8 and 465.6/1,000 livebirths. The frequency of RDS was higher in males than in females and the corresponding relative risk, RR, was 0.7, with 95% confidence interval, CI, ranging from 0.5 to 0.9. The risk of RDS markedly increased with decreasing birth weight: compared to babies weighing more than 2500 g at birth the RR estimates were respectively 1.4, 4.5, 8.8 and 39.3 in those weighing > 2000-2500 g, > 1500-2000 g, > 1000-1500 g and 1000 g or less. Likewise, compared to babies born between the 35th and the 37th week of gestation, the RR of RDS was 3.3 and 21.5 in those born between the 31st-34th or before the 31st week of gestation. Multiple pregnancy, gestational or chronic diabetes, pregnancy-induced or chronic hypertension and premature rupture of the membranes were not related to the risk of RDS.(ABSTRACT TRUNCATED AT 250 WORDS)